Edit entity provider page: Duplicate objects should not be allowed in the form

Bug description

Duplicating objects should not be allowed in the form on edit page of entity provider

How to reproduce the issue

1. Create Entity Provider and go to edit page
2. Add duplicated values to the form, for example:
3. In Assertion Context > Singing And Encryption > Certificate Aliases > Signing - Optional add the same value twice

Expected behaviour

Should not be able to add same value twice

Current behaviour

Able to add same value twice